When constructing down choppers, it is often necessary to provide two or more power modules in parallel, with each of the modules providing a given amount of current, such as 200 amperes. In the past, each of the power modules is connected in parallel and requires its own choke to create the desired output inductance for controlling current flow through the arc and modulating the input pulses to the welding operation. When separate chokes were heretofore used in the output of each power module in a multi-stage D.C. chopper, certain difficulties were presented. Since the output chokes of the various stages were in parallel, the effective inductance of the down chopper was substantially reduced. Consequently, there was a rapid rise of short circuit current, which rate of rise could exceed 300 amperes/msec. This rapid rise of the current at the output of the down chopper produced a very harsh, drastically pinched arc and caused substantial spatter. Thus, the use of the two or more parallel power modules in a D.C. chopper, of the type to which the present invention is directed, were not adapted for smooth and controlled arc welding.
As another background concept, prior D.C. choppers used for welding had variable output chokes, or inductors, to control the rate of rise of short circuit current. By adding more inductance into the output circuit of the chopper, there was a slower rate of rise. This slower rate of rise results in a softer, more fluid arc, with less spatter. The faster rate of rise, which is caused by decreased effective inductive reactance when using multi-stage down choppers, results in a harsher more penetrating arc with more spatter. If the inductance is caused to decrease by a large magnitude, the arc current may be insufficient to maintain the arc, which can result in an unstable arc in the welding operation. Problems associated with an inductor arrangement having low inductance is especially troubling at extremely low wire feed speeds. To correct problems associated with high or low inductance values in a down chopper, the welding industry has generally adopted inverters so that a premonition circuit can sense an impending short and initiate a current ramping circuit to control the rate of rise of the current electronically without substantial dependence upon the value of the inductance in an output choke. Use of an inverter, with electronically controlled current pulses to control the welding operation, has resulted in some difficulty in that the system was not able to sense the exact time of a short, especially when long cables were used for the welding operation.
The disadvantages associated with parallel output chokes in a multi-stage down chopper and the vicissitudes of the output chokes when used to control the slopes of rising current and decreasing current at the output of the down choppers are still problems which plague the down chopper and cause hesitation in the adoption of a down chopper for welding. Down choppers often require two or more parallel power modules or switching stages to obtain the necessary output capability. This demand results in the problems associated with parallel operation of switching device inductors. In addition, down choppers have no procedure for electronically controlling the pulse shape of the wave form in a welding cycle, so the problems associated with attempting to adjust the output inductor or inductors to control either the rise or fall of the welding current have not been resolved. All of these problems have contributed to the preference of inverters over less expensive, more rugged and maintenance free down choppers.